1. Field of the Invention
This invention relates to the field of beam scan velocity modulation for cathode ray tubes in televisions and the like.
2. Description of Related Art
Beam scan velocity modulation (BSVM) can be used in color television sets, for example large screen television sets and/or television sets with sophisticated display features and high resolution, to improve horizontal resolution of the picture. FIG. 1 illustrates a block diagram of a deflection circuit 10 using a basic BSVM circuit according to the prior art. The circuit 10 has a main luminance processing path comprising an RGB decoder 12, a video amplifier 14 and a picture tube 16. The picture tube 16 has a main deflection yoke 18 and an auxiliary deflection yoke 20. Beam scan velocity modulation is provided by a differentiator 22 and an amplifier 24. The luminance signal is differentiated and applied to the auxiliary yoke 20 mounted on the neck of the picture tube 16 under the main yoke 18. The horizontal deflection field is modulated by the current in the auxiliary yoke, and hence the scan velocity is modulated. A black to white transiton followed by a white to black transition for a luminance signal is shown in FIG. 2(a). The auxiliary yoke current shown in FIG. 2(b) increases during the first part of the leading edge of the black to white transition, and decreases during the second part of the black to white transition. A corresponding but negative auxiliary yoke current flows during the white to black transition. The effects of the auxiliary deflection current on the deflection and scan velocity are shown in FIGS. 2(c) and 2(d) respectively. An increase of scan velocity reduces brightness while a decrease raises brightness on the screen. The result is a rapid change in image brightness distribution so that a transition appears sharper, as shown in FIG. 2(e), where the solid line shows brightness with BSVM and the dashed line shows brightness without BSVM. The much sharper transition is evident for BSVM. The advantage of BSVM lies in emphasizing image contours without increasing beam current. The spot size is therefore not enlarged.
The disadvantage of such a simple system is that visible sharpness improvement is only obtained for high resolution test pictures and text. This is due to the differentiator, which emphasizes the higher frequencies. While high resolution pictures are enhanced, pictures of normal or low resolution, which particularly need improvement, are not enhanced.